In the prior art, usually more than one input line of a microprocessor or microcontroller is required for the detection of closure of one of two or more switches. Generally, a keypad having n2 buttons or switches arranged in an n×n matrix will require 2N number of input/output lines for decoding switch closures on the keypad. For example a two-by-two keypad requires four input and output (I/O) lines for determining a key press and a three-by-three keypad requires six I/O lines. Such a matrix configuration works if the required number of I/O lines is available. However it is not always possible to provide the required number of I/O lines due to either space or cost constraints.
Examples of devices where cost and size are important design considerations are portable music playback devices such as CD players and more recently palm-sized personal computer (P/PC). As these devices are meant to be carried around, it is highly desirable to reduce the size of these devices. Reducing the number of I/O lines required is one way of achieving a smaller device.
Another factor which dictates that less I/O lines be used in such devices is the types of standard jacks available for connecting a pair of earphones to the device. Standard jacks are either of the three or four-pole types. A three-pole jack has three sections or poles, two of these poles are used for the left and right sound channels and the last pole for a ground (GND) connection. Such a three-pole jack suffices for connecting a pair of stereo earphones to the device. To support anything more than the pair of stereo earphones, more connections than what a three-pole jack can support are required.
For example, 4-pole jacks have become a standard part when remote controls are introduced. When used to play music, these devices are usually carried on the waist or in a bag rendering the control buttons on the device difficult to access. To increase usability, manufacturers of these devices have introduced a remote control which is attached to the wires connecting the earphones to the device. This remote control is small in size and has a few control buttons—play, stop, forward and reverse. When a user puts on the earphones, the remote control is conveniently located at chest level and therefore allows the user easy access.
With a recording device such as a P/PC, it will also be convenient to locate a microphone on the remote control. With the microphone added onto the remote control, more connections are required to the device. These connections would require jacks to have more than four poles, preferably five poles. However, five-poles jacks are currently non-standard and would have to be specially manufactured. Cost involved in tooling such a non-standard jack is prohibitive. One possible solution is to use two separate standard jacks to support the combination of the remote control, the earphones and the microphone. However, such a solution would impede the usability of the device and would require more space on the device than would a single jack.
From the foregoing, the prior art therefore has a need for an innovative method and apparatus for detecting the closure of one of a plurality of switches using one input pin. The method also uses the same input pin to receive an audio frequency signal after detecting that a particular recording switch is closed. With this apparatus, a standard four-pole jack will be sufficient for connecting a remote control, a pair of earphones and a microphone to an audio recording and playback device. It is advantageous to use such a four-pole jack as the device would then be able to operate with a pair of earphones that is attached to a standard three-pole jack.